Method and means for compensating thermal lensing in a laser system

ABSTRACT

A negative thermal circular lens is provided for compensating the thermal lensing effect inherent in output radiation generated by a solid-state laser rod of the annular configuration. The compensating thermal lens is of annular construction with metallic coatings on its inner and outer cylindrical surfaces. These coatings are heated by RF energy to develop a thermal gradient in the direction to induce stress resulting in a negative lensing effect so that the positive lensing effect in the annular laser output beam is substantially canceled. This invention relates generally to a method and means for compensating thermal lensing in a laser system and more particularly to the compensation of laser radiation generated by solid-state-annular-type laser rods.

United States Patent [54] METHOD AND MEANS FOR COMPENSATING THERMALLENSING IN A LASER SYSTEM 7 Claims, 3 Drawing Figs.

[52] US. Cl 331/945, 350/ l 75 GN [51] Int. Cl H015 3/00 [50] Field ofSearch 331/945;

350/96 WG, 160, 175 ON, 179

[56] References Cited UNITED STATES PATENTS 3,399,012 8/1968 Peters360/96 3,484,714 12/1969 Koester et a1 331/1945 3,531,185 9/1970Bucksbaum 350/179 LIGHT PUMP SOURCE OTHER REFERENCES Akhmanov et al.,lEEE low. of Quantum Electronics, Vol. QE-4, pp. 568- 575, Oct. 1968Primary ExaminerRonald L. Wibert Assistant Examiner-Edward S. BauerAttorney-Pastoriza & Kelly ABSTRACT: A negative thermal circular lens isprovided for compensating the thermal lensing effect inherent in outputradiation generated by a solid-state laser rod of the annularconfiguration. The compensating thermal lens is of annular constructionwith metallic coatings on its inner and outer cylindrical surfaces.These coatings are heated by RF energy to develop a thermal gradient inthe direction to induce stress resulting in a negative lensing effect sothat the positive lensing effect in the annular laser output beam issubstantially canceled.

This invention relates generally to a method and means for compensatingthermal lensing in a laser system and more particularly to thecompensation of laser radiation generated by solid-state-annular-typelaser rods.

PATENTED SEP28 I971 R F CONTROL LIGHT PUMP SOURCE RADlUS Ts TEMPERATUREF I G. 3

WALTER KOEXfiQE ATTORNE VS METHOD AND MEANS FOR COMPENSATING THERMALLENSING IN A LASER SYSTEM BACKGROUND OF THE INVENTION Under theinfluence of optical pumping, thermal gradients are introduced incrystalline and glass-type laser materials. The change in temperaturewithin a laser rod causes a thermal distortion of the laser beam due toa temperatureand stress dependent variation of the refractive index. Inaddition, the stresses cause an elongation of the rod. As a result ofthese effects, the laser rods exhibit thermal lensing; that is, theoutput beam converges in a generally positive manner the same as thoughuniform radiation were passed through a convex lens.

In many applications, a beam-expanding telescope is required to decreasethe beam divergence of the system. The positive thermal lensing effectexhibited by a solid laser rod can be compensated for by adjusting theseparation of the two lenses of the telescope. Such an adjustment ispossible because thermal lensing in a solid rod can be compensated forby a negative spherical lens, at least to a first-order approximation.

However, for certain special applications, laser rods such as thoseformed from ruby are of an annular configuration defining essentiallyinner and outer cylindrical surfaces. In the case of such an annularrod, the thermal lensing resulting is equivalent to the focusingproperties of a doughnut-shaped positive lens, referred to generally asa positive circular lens. Accordingly, the compensating optical devicehas to be a negative circular (doughnut-shaped) lens. Such anegativetype circular lens having nonspherical surfaces is extremelydifficult to fabricate in an optical shop.

BRIEF DESCRIPTION OF THE PRESENT INVENTION With the foregoing in mind,the present invention contemplates a method and means for compensatingthermal lensing in annular-type laser devices which avoids the necessityof fabricating nonspherical surfaces.

More particularly, the invention comprises a cylindrical body havingoptically flat opposite end surfaces. This body is provided with aheating means thermally coupled to the body for developing a thermalgradient which creates a lens effect constituting the opposite of thelens effect inherent in the laser radiation. By this means, not only canthe desired compensation be realized but the negative thermal circularlens itself is "thermally tunable" in that its focal length may bevaried by controlling the degree of heat supplied to the lens.

BRIEF DESCRIPTION OF THE DRAWING A better understanding of the inventionwill be had by referring to the accompanying drawings in which:

FIG. 1 is a schematic showing of a laser system with which the negativethermal circular lens of the present invention is used for eliminatinglens effect inherent in the output radiation from the system prior topassing into an expanding lens system;

FIG. 2 is an enlarged cross section of the negative circular lens takenin the direction of the arrows 22 of FIG. 1; and,

FIG. 3 illustrates the temperature variation with radial distancechanges from the annular central portion of the lens body of FIG. 2towards its outer and inner cylindrical surfaces.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. Ithere is shown a laser rod with a central opening defining an annularconfiguration. The rod 10 is light pumped by an helical flashlamp 11powered from a source 12. Suitable end mirrors l3 and 14 in opticalalignment with the rod 10 define the optical cavity for the laser. Lightpumping of the rod 10 by the flashlamp 11 builds up an invertedpopulation level in the laser ions in the rod and at threshold, thelaser ions in the upper energy level fall back to a lower levelresultingin simulated emission of radiation which is enhanced throughregenerative action taking place between the end mirrors 13 and 14. Theend mirror 14 may be partially transmissive in order to couple the beamout of the optical cavity.

Nonnally, the annular laser rod 10 and helical flashlamp l l aresurrounded by an enclosure or head through which cooling liquid iscirculated. The cooling of the rod is effected at both the inner andouter cylindrical surfaces resulting in a temperature gradient whichdecreases from the annular central portion of the laser rod body inradial directions towards the inner and outer cylindrical surfaces. Thischange in temperature, as described heretofore, causes a thermaldistortion of the laser beam due to the stress dependent variation ofthe refractive index and further stresses resulting in an elongation ofthe rod. As a consequence, the annular output laser beam exhibits apositive lensing efi'ect as indicated at B1 in FIG. I.

In accord with the present invention, a negative thermal circular lens15 is introduced in optical alignment with the output annular beam Bl.Heating control of the compensating circular lens 15 is effected by anRF control 16 to induce a negative lensing effect in the circular lens15 thereby resulting in cancellation of the positive lensing effect toprovide a compensated output beam B2. This beam may then be passedthrough a conventional beam extending lens system or telescope 17 todecrease the beam divergence of the system.

FIG. 2 illustrates details of the preferred embodiment of the negativecircular lens 15 of FIG. 1. As shown, the compensating lens comprises acylindrical member 18 having optically flat end surfaces 19 and 20 and acentral opening to define an annular body. This body is dimensioned tointercept the annular laser beam B] when the device is used with thelaser system of FIG. 1.

The outer and inner cylindrical surfaces are provided with metalliccoatings 21 and 22 which may constitute gold. An insulating material 23surrounds the outer coating 21 as shown. The insulation coating 23supports a plurality of RF coils 24 supplied with radiofrequency energyfrom the RF control 16 of FIG. 1.

FIG. 3 illustrates at 25 the temperature gradient developed in the body18 of FIG. 2 when the cylindrical surfaces are heated. As shown, thetemperature is minimum at the annular central portion of the body andincreases radially outwardly towards the outer and inner cylindricalsurfaces.

OPERATION In operation, the negative circular thennal lens 15 ispositioned in optical alignment with the output radiation as illustratedin FIG. 1. Radiofrequency energy is then supplied to the coils 24 toestablish an RF field electromagnetically coupled to the coatings. Thecoatings themselves will generate heat at the cylindrical surfaces ofthe body because of the dielectric losses in the coatings. In thisrespect, the outer coating 21 is adjusted in thickness so as to allowthe RF field to penetrate to the inner coating 22.

The heat generated in the two metallic coatings will cause a thermallens effect in the body 18 as a consequence of the thermal gradient asdescribed in FIG. 3. This thermal gradient changes the optical pathlength according to the following equation:

AP=dAn+(nl )Ad where d is the thickness of the disc and n is the indexof refraction. The second term in the above equation takes into accountthe thermal elongation of the disc.

An represents the total change in the index of refraction. This changein the index of refraction arises from two different effects: the firstis a temperature-dependent change of index of refraction and the secondis a stress-dependent change of the index of refraction. With a disc ofoutside diameter of 1.3 centimeters and inside diameter of 0.63 of theoutside diameter, and of length equal to 5 millimeters, a thermalgradient of Centigrade can be generated between the outer and innercylindrical surfaces. Under these conditions, an effective negativefocal length of about 4 meters results. This negative lensing can beused to compensate for the positive thermal lensing of the laser rod ofFIG. 1.

By adjusting the degree of heating and thus the temperature gradient,the focal length of the circular compensating lens can be varied toaccommodate different positive lensing effects in various laser rods.When the negative thermal lens is used, conventional optics can then beemployed for the beam expanding telescope.

Waht is claimed is:

l. A method of compensating thermal lensing in a laser system comprisingthe steps of:

a. passing laser radiation from said system through a lens material; and

b. heating said lens material to thereby develop a thermal gradienttherein which creates a lens effect constituting the opposite of thelens effect characterizing said laser radiation.

2. The method of claim 1, including the steps of varying the heatingsupplied to said lens to thereby vary the focal length of the createdlens effect so that it may be adjusted to compensate for changes in thelens effect characterizing said laser radiation.

3. A thermal lens for compensating the lens effect inherent in laserradiation comprising, in combination:

a. a cylindrical lens body having opposite flat end surfaces;

and

b. heating means thermally coupled to said body for developing a thermalgradient which creates a lens effect constituting the opposite of saidlens effect inherent in said laser radiation.

4. A thermal lens according to claim 3, in which said heating meansincludes: at least one metallic coating on the peripheral portions ofsaid body; and a plurality of RF coils in electromagnetic couplingrelationship with said coating whereby heat is developed in said coatingby said RF coils.

5. A thermal lens according to claim 4, including means for adjustingthe RF field strength generated by said coils to thereby control thedegree of heating of said coating so that the thermal lens effectcreated in said body can be varied to vary the effective focal length ofthe thermal lens.

6. A laser system comprising, in combination:

a. a solid-state laser rod of annular configuration defining inner andouter cylindrical surfaces;

b. light-pumping means for said rod;

c. end mirrors defining an optical cavity so that stimulated emission ofradiation can take place, said radiation being in the form of an annularbeam exhibiting a positive lensing effect as a consequence of heating ofsaid rod by said light-pumping means; and

d. a negative thermal circular lens positioned in optical alignment withsaid annular beam comprising:

1. a cylindrical member having flat end surfaces and a central openingto define an annular body dimensioned to intercept said annular beam;

2. metallic coatings on the outer and inner cylindrical surfaces of saidbody;

3. an outer cylindrical covering of insulating material overlying thecoating on the outer cylindrical surface;

4. a plurality of RF coils surrounding said insulating material; and

5. RF generating means connected to said coils for establishing an RFfield electromagnetically coupled to said coatings to heat the same toestablish a thermal gradient on said body in a direction to inducestress resulting in a negative lensing effect whereby said positivelensing effect of said annular beam is substantially cancelled.

7. A laser system according to claim 6, in which the temperaturegradient of said annular laser rod is from its annular central portionradially outwardly towards its inner and outer cylindrical surfaces andthe temperature gradient in said negative thermal circular lens is fromits inner and outer cylindrical surfaces towards its annular centralportion.

2. The method of claim 1, including the steps of varying the heatingsupplied to said lens to thereby vary the focal length of the createdlens effect so that it may be adjusted to compensate for changes in thelens effect characterizing said laser radiation.
 2. metallic coatings onthe outer and inner cylindrical surfaces of said body;
 3. an outercylindrical covering of insulating material overlying the coating on theouter cylindrical surface;
 3. A thermal lens for compensating the lenseffect inherent in laser radiation comprising, in combination: a. acylindrical lens body having opposite flat end surfaces; and b. heatingmeans thermally coupled to said body for developing a thermal gradientwhich creates a lens effect constituting the opposite of said lenseffect inherent in said laser radiation.
 4. A thermal lens according toclaim 3, in which said heating means includes: at least one metalliccoating on the peripheral portions of said body; and a plurality of RFcoils in electromagnetic coupling relationship with said coating wherebyheat is developed in said coating by said RF coils.
 4. a plurality of RFcoils surrounding said insulating material; and
 5. RF generating meansconnected to said coils for establishing an RF field electromagneticallycoupled to said coatings to heat the same to establish a thermalgradient on said body in a direction to induce stress resulting in anegative lensing effect whereby said positive lensing effect of saidannular beam is substantially cancelled.
 5. A thermal lens according toclaim 4, including means for adjusting the RF field strength generatedby said coils to thereby control the degree of heating of said coatingso that the thermal lens effect created in said body can be varied tovary the effective focal length of the thermal lens.
 6. A laser systemcomprising, in combination: a. a solid-state laser rod of annularconfiguration defining inner and outer cylindrical surfaces; b.light-pumping means for said rod; c. end mirrors defining an opticalcavity so that stimulated emission of radiation can take place, saidradiation being in the form of an annular beam exhibiting a positivelensing effect as a consequence of heating of said rod by saidlight-pumping means; and d. a negative thermal circular lens positionedin optical alignment with said annulaR beam comprising:
 7. A lasersystem according to claim 6, in which the temperature gradient of saidannular laser rod is from its annular central portion radially outwardlytowards its inner and outer cylindrical surfaces and the temperaturegradient in said negative thermal circular lens is from its inner andouter cylindrical surfaces towards its annular central portion.